In recent years, filtration membranes (separation membranes) are utilized in a variety of areas such as the field of water treatments involving beverage manufacturing, water purification treatment and waste water treatment, as well as food industry field, and the like. Filtration membranes have been utilized for eliminating impurities in water as an alternative to conventional sand filtration or coagulating sedimentation process in the field of water treatments involving beverage manufacturing, water purification treatment, waste water treatment and the like. Moreover, in the food engineering field, filtration membranes have been utilized for the purpose of concentrating liquids, and separating and eliminating yeast etc., used in fermentation.
Since the quantity of treated water with filtration membranes used in various ways as described above is large in the field of water treatments such as water purification treatment and waste water treatment, improvement of water permeability has been demanded. Superior water permeability enables the membrane area to be decreased, and thus compact apparatuses can be provided to permit cost reduction of equipment, leading to advantages in terms of costs for replacing the membrane as well as the area of equipment.
Additionally, in waste water treatments, a microbicide such as sodium hypochlorite is charged into a membrane module portion for the purpose of sterilizing charged water and preventing biofouling of the membrane. Furthermore, since the membrane itself is washed with an acid, an alkali, chlorine, a surfactant etc., filtration membrane requires chemical resistance properties.
Moreover, in production of tap water, troubles of contamination of treated water with pathogenic microorganisms that are resistant to chlorine such as cryptosporidium, derived from feces and urine of livestocks and the like are not dealt with in a water purifying plant have elicited since 1990s. Thus, for preventing such troubles, sufficient separation characteristics so as to avoid contamination of treated water with the raw water, and great physical strength have been required for filtration membranes.
Therefore, filtration membranes require superior blocking performance, chemical resistance, physical strength, water permeability and fouling resistance. Thus, filtration membranes produced using a polyvinylidene fluoride based resin having both chemical resistance and physical strength in combination have been used. However, since filtration membranes produced using a polyvinylidene fluoride based resin is hydrophobic, fouling substances are likely to be attached to pores of the filtration membrane, and thus washing with a chemical such as sodium hypochlorite must be frequently carried out. Therefore, lifetime of the membrane is shortened, and the frequency of replacement of the membrane is increased, leading to a problem of high running costs. In addition, since filtration membranes produced using a polyvinylidene fluoride based resin contain halogen molecules, environmental endocrine disrupters are generated upon incineration for disposal, which may lead to problems of great environmental burden.
On the other hand, cellulose based resins have attracted attention in addition to polyvinylidene fluoride based resins. Cellulose based resins are more hydrophilic as compared with polyvinylidene fluoride, and is advantageous in high fouling resistance. Additionally, since halogen is not contained, it is advantageous in less environmental burden. However, it is disadvantageous in low physical strength.
Furthermore, a polyethersulfone (hereinafter, may be abbreviated as “PES”) is noteworthy as a compound that exhibits intermediate characteristics of polyvinylidene fluoride based resins and cellulose based resins in both terms of physical strength and fouling resistance (for example, see Patent Documents 1 to 3). However, even when a polyethersulfone is used as a filtration membrane, hydrophilicity of the membrane itself is still insufficient, and a filtration membrane which is satisfactory in light of fouling resistance has not been obtained under current circumstances.